1. Field of the Invention
The present invention relates to a chain lever hoist that raises or lowers a load by means of a manually operated lever action and has a feature to allow its chain to run freely.
2. Description of the Related Art
A chain lever hoist must offer free-running operation of its chain wheel in shave, sometimes called a "load sheave", to allow for the free running of the chain, in addition to raising or lowering operation (hereinafter referred to as upward or downward winding operation), of the load chain by means of a lever action.
FIG. 13 shows one type of conventional chain lever hoist known as a spring chain lever hoist that comprises a mainframe (I), a load sheave (Ro) freely rotatably supported by the mainframe (I), a spindle (Ha) supported by the mainframe (I) in a manner that allows the spindle to integrally rotate with the load sheave (Ro), a fixed friction plate (Ni) secured to the spindle (Ha), a hub (He) into which the spindle (Ha) is screwed and which is rotated by a lever (Ho) that is pivoted around the spindle (Ha), a ratchet gear (To) with two brake linings (Ti, Ti) arranged respectively on each side of the rachet gear (To) between the fixed friction plate (Ni) and the hub (He) in such a manner that the ratchet gear (To) rotates freely around the spindle (Ha), and a ratchet pawl (Ri) that is mounted on the mainframe (I) in such a manner that the ratchet pawl (Ri) engages with the ratchet gear (To) to be allowed to rotate only in the direction of the upward winding operation. The distance between the fixed friction plate (Ni) and the hub (He) can be adjusted or varied by rotating the hub (He) i.e. screwing it more or less onto the spindle (Ha), in such a way as to squeeze or release the ratchet gear (To) and the brake linings (Ti, Ti). Furthermore, as an assist mechanism, a coil spring (Nu) is disposed between the fixed friction plate (Ni) and the hub (He) so that the force of the coil spring (Nu) constantly, urges the hub (He) outwardly (to the right-hand side in FIG. 13), thereby reducing the contact pressure between the brake lining (Ti) and the hub (He), and thus decreasing the braking effect.
A discussion of the operation of the conventional spring chain lever hoist follows. To carry out an upward winding operation, a switch pawl (Ru) first operated to engage with a switch gear (Wo) of the hub (He) in the upward winding direction, whereby the hub (He) is allowed to rotate only in the upward winding direction. The lever (Ho) is pivoted in the upward winding direction, and the torque generated by a suspended load on the chain causes the hub (He) to be screwed inwardly along the spindle (Ha) in such a manner that the hub (He) and the fixed friction plate (Ni) squeeze the ratchet gear (To) and the brake linings (Ti, Ti) therebetween. Thus, the torque of the lever (Ho) is transmitted from the hub (He) to the load sheave (Ro) via one brake lining (Ti), the ratchet gear (To), the other brake lining (Ti), the fixed friction plate (Ni), and the spindle (Ha). The ratchet gear (To) rotates with the ratchet pawl moving or oscillating thereon, causing the load sheave (Ro) to rotate in the upward winding direction. The load is thus lifted up.
To carry out a downward winding operation, the switch pawl (Ru) is operated to engage with the switch gear (Wo) in the downward direction, whereby the hub (He) is allowed to rotate only in the downward winding direction. The torque generated by a suspended load causes the hub (He) to be screwed inwardly along the spindle (Ha) in such a manner that the hub (He) and the fixed friction plate (Ni) squeeze the ratchet gear (To) and the brake linings (Ti) therebetween in the same manner as in the upward winding operation. When the lever (Ho) is pivoted in a downward winding direction, however, the squeezing force is decreased by the torque of the lever (Ho). Thus, the fixed friction plate (Ni) slides against the ratchet gear (To), and the fixed friction plate (Ni), the spindle (Ha) and the load sheave (Ro) rotate in the downward winding direction in accordance with the degree of rotation of the hub (He). Thus, the load is lowered.
To carry out a free running operation, the switch pawl (Ru) is operated to disengage from the switch gear (Wo), whereby the lever (Ho) is disengaged from the hub (He). The hub (He) is rotated to be spaced apart from the fixed friction plate (Ni), and the contact pressure of the hub (He) against the ratchet gear (To) and the brake linings (Ti, Ti) is reduced. The coil spring (Nu) urges the hub (He) outward, rendering the braking action ineffective. Thus, the load sheave (Ro) is allowed to rotate freely.
In lever hoist of this type described above, it is common for a heavy duty hoist with a load capacity of 0.5 tons or more to have its load sheave and spindle linked through a plurality of reduction gears as shown in FIG. 13. However a light hoist with a load capacity of less than 0.5 tons commonly has its load sheave and spindle connected to each other directly.
In the above-described conventional spring lever hoist, the following problems arise. The assist mechanism comprising coil spring (Nu) blocks the screwing control position of the hub from returning to the winding operation position, when the screwing control position of the hub (He) relative to the spindle (Ha) is shifted. To perform the above operation, the hub (He) must be rotated while the chain is held by hand, or the spindle (Ha) must be rotated while the lever (Ho) is engaged with the hub (He) (typically by allowing the switch pawl (Ru) to engage with the switch gear (Wo) of the hub (He)). The first method permits an immediate switching to the free-running operation, but an operator must use both hands and gets his hands dirty in the operation. The second method allows a single-handed operation, but the operation is a two-step sequence because a locking engagement step is involved. In the second method, the rotation of the spindle may be carried out by pulling the chain. This option may be useful when the spindle (Hal is out of reach of the operator's hand. However, the operator is not freed from getting his hands dirty, and the operation remains a two-step sequence. Also, the operator cannot judge the operation position or mode by just looking at the switch pawl, because the appearance of the switch pawl does not show the condition of whether the contact pressure of the hub against the ratchet gear and the brake linings is reduced or not.